There are about a dozen answers to your question.
The Barbie and family items from 1959 to 1972 were tagged. Usually it was the main piece. For instance, if it came with both a coat and a dress, one would have a tag and not the other. Or if it was a shirt and slacks or a skirt, usually just the shirt had the tag. All of these tags were white cloth, with the doll name sewn in black script (Barbie, Ken, Francie, Skipper, Tutti, etc.)
There were no tags at all from 1973 to 1976.
From 1977 to sometime in the late 80s, the tags were white paper with the name in black printed on it. By that I mean it was both imprinted and the name was in block letters. In the 1990s, for a brief time, the tags were square purple paper with the name "Barbie" arranged in four blocks. (I really liked the purple tags!). The placement of the tags were the same as earlier--one tag per ensemble.
I can't tell you about post-2000 since I don't collect new stuff any more. I have noticed that many items of the past 15 years, even if it's for Skipper or Ken, just has the Barbie tag inside. (Just to confuse people?)
There's another way to identify genuine Mattel. In the 1960s, the outfits closed with small silver snaps or a hook with a thread loop. In the 1970s many outfits had a small button in the back with a thick string (almost like rickrack) loop. In the 1980s, the snaps returned, and in the late 1980s, Mattel used white plastic square snaps, which have been my favorite of all the types of closures. Since the 1990s it's been Velcro, which I despise.
You will find thousands of loose fashion doll items with a tag that says "Made in Hong Kong." These are not Mattel. Usually the company was Shillman or Mego or one of their umpteem short-lived sister factories. Many of these are nicely made, but they are considered competition.
